Hydrated lime, hereinafter termed `lime` (calcium hydroxide) in distinction from quicklime (calcium oxide), its industrial precursor, is traditionally used in wastewater conditioning. It is desirable to supply lime to the user in the form of an aqueous suspension, especially by means of a tanker filled and emptied by pumping. Hitherto it has hardly been possible to pump lime suspensions at solids contents above 35% by weight. This involves costs in transporting water which have in the past made such operations uneconomic.
In industrial practice it is known to treat aqueous particulate systems which polyelectrolytes in order to promote the flocculation of the particles. In the treatment of wastewater this enables clarification by sedimentation of the suspended matter to occur more rapidly and effectively than without the polyelectrolyte addition. In the treatment of wastewater these polyelectrolytes are ofen used in conjunction with a conditioning aid or destabiliser such as hydrated lime.
In wastewater treatment the use of cationic polyelectrolytes is now generally preferred, because they are directly substantive to organic suspended matter in most wastewasters in sewage, animal farm waste, papermaking effluent and many industrial effluents, suspended matter carries a negative charge and is thus best treated by cationic polyelectrolytes which carry a positive charge.
In the treatment of potable water, lime is used as a part of a lime or lime-soda water-softening process; polyelectrolyte may be added to accelerate the sedimentation of suspended organic or inorganic matter and of the calcium carbonate particles precipitated by interaction of the lime with the hardness of the water.
In the above processes concerned with wastewater treatment it is advantageous to add a weighting agent which, by increasing the specific gravity of the flocculated matter, increases the rate of sedimentation and thus the rate of clarification of the liquor.
In wastewater treatment it is customary to add conditioning aids, such as lime, and polyelectrolyte separately; weighting agents where used are also separately added.
In all industrial practice it is clearly desirable for additives, instead of being added batchwise or metered into a system separately, to be added together as one easily pumped and easily metered aqueous suspension containing all the ingredients. Duplication of pumps and metering equipment is thus avoided.
It would also be industrially convenient for such an additive to be manufactured at a central point such as the plant that manufactures one or more of the ingredients. This additive could then be transported, as a suspension, by tanker or other means to the user site. This would avoid the necessity for the user to employ plant for preparing aqueous suspensions or solutions of the various materials or chemicals. In particular it is unpleasant to prepare suspensions from dry lime because of dusting into the atmosphere of fine irritant particles. The preparation of solutions of polyelectrolytes is also not free from difficulty and is best carried out centrally with appropriate equipment.
For this operation to be commercially successful it would be necessary for the material carried by tanker to be of high solids content so as to minimise transport charges on water.
It is clearly of the greatest convenience to the user for a single additive suspension/solution to be supplied by road or rail tanker and cleanly pumped into a tank having simple facilities for avoiding sedimentation, e.g. by slow stirrer or bubble suspension.
In the treatment of sewage and the like, various types of apparatus are used for the settling and clarifying processes. The object of these processes is to enable particles in suspension in water, that is those particles already existing in the raw water and/or those resulting from the action of a chemical reagent (added to effect, for example, coagulation, removal of iron or chemical purification), to sink to the bottom. It is possible to couple a static settling tank to a flocculation unit in such processes. However, when the treatment comprises the addition of reagents, it can be an advantage to combine flocculation and clarification within a single unit, as this permits complete reaction to be obtained with the formation of dense precipitates. Sludge-blanket clarifiers are commonly used.
In French Patent Specification No. 586,506 (published 25 March 1925), it is disclosed that effluent water, such as sewage, may be treated with a small quantity (generally 0.05 to 0.5%) of lime to give flocculant precipitate that can be drained and then pressed to give a compact mass. An absorbent material, for example calcium carbonate, may be added to the lime.
Recently, P. Stead and B. Winfield, of Portsmouth Polytechnic, England, have developed a "once through" plant for the rapid sedimentation of primary sewage (i.e. sewage that has passed through grit tanks) using a treatment with lime and Zetag 92 (a cationic polyacrylamide from Allied Colloids Ltd., England). The treated sewage flocculates rapidly. In a correctly designed cone-shaped treatment tank with a central downcomer, a stable sludge blanket can be formed and the effluent from the system can be maintained at a turbidity of less than 30 nephelometric units; this is accompanied by a substantial reduction in organics, phosphates and the like.
The system developed by Stead and Winfield will be of particular advantage in areas where at present virtualy untreated sewage is pumped into the sea or into tidal estuaries. The plant should enable a massive reduction in pollution to be achieved at minimal cost. However, it would clearly be desirable to have available a single additive composition that could be employed as a "one shot" treatment in the "once through" plant.
It is know that sewage, particularly in hard water areas, has a lime demand (or lime charge); typically, the lime demand of sewage in a hard water area is in the range of 180 to 240 ppm. Reduction in the lime charge used for treatment of the sewage gives poor results, in particular poor clarification and failure to remove organic matter. The addition of calcium carbonate, alone or with a polyacrylamide, again is known to give poor results in the treatment of sewage.
When calcium carbonate is precipitated in sewage in situ, using hydrated lime, either from calcium bicarbonate naturally present in the water or from calcium bicarbonate that is produced by the addition of calcium chloride or sodium bicarbonate, poor flocs and a hazy supernatant liquor are formed. The cause of this is believed to be the rapid absorption of proteins by the calcium carbonate formed, this mechanism inhibiting crystal growth and floc formation.
As mentioned above, it is known to use lime and Zetag 92 for the treatment of sewage. However, it would clearly be desirable to improve yet further the rate of sedimentation and the strength of the resultant floc.
Any process which improves the rate of flocculation of suspended particles and increases their density and strength would be particularly applicable to processes of sludge blanket clarification.
It will be seen from the above discussion that there is a need for suspension of lime (or other alkaline earth metal hydroxide) of high solids content and which possesses a rheology that makes it suitable for tanker transport. In a number of applications it would be desirable for such a suspension also to contain a weighting agent and/or to be capable of accommodating a cationic flocculating agent of high molecular weight without unacceptable deterioration in the rheological properties of the suspension.